14 July 2010

Life and death and sex choices in mantids

If ever there was a time to be careful about who you were going to mate with, it would probably be when there was a good chance you were going to die in the attempt.

And we’re not talking about some sort of heroic situation where the male has to endure hardships to get to the female. We’re talking about situations where the female herself is the threat.

“Fair princess, I have arrived to...”

CHOMP. Nom nom nom.

A lot has been written about the cannibalistic tendencies of praying mantises. Indeed, a recent New Scientist article called male mantids “the poster boy of risky sex.” Why the female tend to eat their mates seems to have a lot to do with them being aggressive hunters that pretty much try to eat anything. But we’re more concerned with the males’ response to that behaviour, rather than worrying about what causes it in the first place.

Barry tests the hypothesis that males pick what females to mate with in part by sensing chemical cues that the female gives off. In part, this is a replication of a previous study in the lab in more natural settings.

To give the males something to choose between, Barry split the females into two groups: one got fed three times as much as the other, so you would expect these would be attractive to males for two reasons. First, they should be in better condition, and second, they should be less hungry, and hopefully less likely to strike out at males.

To prevent the males from using visual cues, the females were placed in cages that were covered. Barry also put out empty control cages (which never attracted males). As expected, the males were significantly more likely to end up in cases containing well fed females.

After the males chose, Barry examined the number of eggs in the ovary to determine fecundity. This is a bit tricky, as you would expect that there’s going to be a correlation between feeding and fecundity, which was the case. But, Barry did find that when males did choose the poorly fed females, they picked ones that had mature eggs. When exposed to eggs alone, however, the males never ended up in those cages.

I suppose one question raised here is: How expensive is it to make a pheromone? Insects are famous for being able to detect and respond to small quantities of pheromones; if so, it may not take much to bring in a mate. Does food restriction shut down the chemical pathway, or are the males very sensitive to concentration levels of pheromones?